Formation of a dry coating film by coating a surface of an object to be coated with a paint containing a prescribed component (a paint composition) followed by drying in order to provide desired characteristics to the surface is a well-known technique. In order to provide desired characteristics by forming a coating film, control of a film thickness of the dry coating film is important. When the formed dry coating film does not have a sufficient film thickness, characteristics to be provided tend to be insufficient.
Control of a film thickness of the coating film, however, is not an easy operation. A conventional method of controlling a film thickness includes i) a method of measuring a thickness of a coating being formed, that is, in a wet state (before drying), with a wet gauge in order to check in advance whether or not a target dry film thickness (for sufficiently obtaining desired physical properties) will be obtained and calculating and checking a dry film thickness based thereon and ii) a method of measuring a film thickness of a dry coating film directly with an electromagnetic coating thickness tester in order to check whether or not a film thickness of a dry coating film has reached a target film thickness.
In any of the methods above, however, it has been virtually difficult to control a film thickness particularly in cases as follows.
1) A case that an area of a surface of an object to be coated which is coated with a paint is large, for example, a case of marine vessels. In this case, it is extremely troublesome and actually difficult to measure a film thickness with a wet gauge or an electromagnetic coating thickness tester over the entire surface of the object to be coated.
2) A case that a surface of an object to be coated which is coated with a paint includes a surface of which coating is not easy. A film thickness tends to vary at the surface of which coating with a paint is not easy. On the other hand, it is often difficult and troublesome to measure a film thickness of a coating film formed on such a surface with a wet gauge or an electromagnetic coating thickness tester.
3) A case that a target film thickness of a coating film formed in a single-coating process is large. As a coating film to be formed in the single-coating process is thicker, a frequency of measurement of a thickness of the coating being formed tends to be often and control of a film thickness becomes troublesome. The “single-coating process” means that a series of operations from coating with a paint until drying treatment is performed once, and therefore, a coating film formed in the single-coating process refers to a coating film formed by coating by application of a paint once or a plurality of times followed by single drying treatment.
As a method for solving the conventional problems as described above, Japanese Patent Laying-Open No. 10-216621 (PTD 1) and Japanese Patent Laying-Open No. 2002-066445 (PTD 2) disclose formation of a coating film with the use as a paint for forming a coating film, of a paint called a “film thickness determination paint” which allows determination as to whether or not a defined film thickness has been reached based on visual observation by a coating worker of change in color difference of a wet coating being formed.